The subject matter of this application is related to the subject matter of British Application No. 0111629.2, filed May 11, 2001, priority to which is claimed under 35 U.S.C. 119 and which is incorporated herein by reference.
1. Field of the Invention
This invention relates to the cooling of electrical machines.
2. Description of Related Art
Electrical machines typically have windings made from insulated conductors in which current flows and which, unless the conductor material is super conducting, have resistive loss. This resistive loss (the so-called I2R loss) heats up the conductor. The continuous rating of the electrical machine generally depends on the temperature limit of the insulation material of the winding. Often these limits conform to internationally recognized values, e.g. Class F (145xc2x0 C.), Class H (180xc2x0 C.) etc., and the expertise of the designer is brought to bear on the problem of removing the I2R loss so as to keep the temperature of the winding below the chosen limit.
The problem is made more difficult because of the generally conflicting requirements to provide good electrical insulation and good thermal conduction. The conductors of the winding are typically coated with an insulating enamel, and the completed winding assembly is typically impregnated with an encapsulating varnish. These measures contribute to ensuring that the electrical insulation of the winding is of a high quality. However, most good electrical insulators are also good thermal insulators, so the use of these materials generally makes more difficult the task of providing paths of high thermal conductivity for the I2R loss from the winding to a heat sink where the loss may be dissipated.
The problem of heat removal is further compounded on electrical machines which have windings in which a coil spans a single tooth, for example brushless dc machines and switched reluctance machines. While the coils of a conventional distributed winding are generally dispersed across a relatively large number of slots and have more intimate contact with the iron of the stator laminations, the short-pitched windings of a salient pole machine generally contact the stator only on two sides of the coil, leading to a heat-removal path of a smaller cross-sectional area.
Various methods are known for improving the efficiency of the dissipation path for the heat in the winding. For example, the coating of encapsulation varnish is sometimes made very thick, so as to eliminate virtually all the air pockets around the winding and provide a path through the varnish for the heat. This improves the heat transfer, but is often a messy and time-consuming process, involving the use of compounds which constitute health and safety hazards.
Liquid cooling has also been proposed. The liquid flow rate can be set to keep the liquid at a relatively low temperature compared with the winding so that the consequential thermal gradient gives high heat transfer from the winding. Cooling by water jackets surrounding the stator core is known. This is often useful when the machine is relatively long compared with its diameter: as the length/diameter ratio reduces, this method is less and less successful and alternative systems are used, for example, coolant pipes introduced into regions of the core and/or windings. Various attempts have been made to put this into practice, e.g. by providing coolant pipes around the winding overhang or through the core itself. FIG. 1 shows a prior art arrangement of a laminated stator 12, of the salient pole type having six poles 14 each surrounded by a coil 16. A rotor 15, mounted to rotate within the stator, has four salient poles 11. The coils on the stator have an overhang region 17 and coil sides 19. Coolant pipes 10 are placed at the ends of the stator core 12, in the plane of the laminations and adjacent to and encircling the winding overhang 17. Typically, these pipes are encapsulated with the winding overhang, but, even so, there is not a good thermal path between the portions of the winding and the slots of the stator and the coolant, since even the best encapsulating resins have a relatively high thermal resistance.
Another proposed method is to use liquid coolant passing through the winding conductors themselves. This technique is more applicable to large machines in the multi-megawatt range, where the great expense of the complex arrangements needed to provide both fluid seals and electrical insulation on the individual conductors is offset by the large saving in the running cost of the machine. Though this technique has been proposed for much smaller machines in very specialized applications, for example, as described in U.S. Pat. No. 5,489,810 (Ferreira), which is incorporated herein by reference, the saving has failed to justify the complexity.
An apparently simpler method would be to pass the coolant through paths which run down the slots in which the winding conductors lie. This is discussed in WO 00/01053 (Sjoberg), which is incorporated herein by reference. This has the advantage of short thermal paths from the winding to the coolant, but it requires electrically non-conducting coolant pipes (which generally also have poor thermal conductivity) and requires the reliably leak proof connection of many pipes to a coolant manifold in a restricted area. FIG. 2 shows a simplified view of this prior art arrangement, with triangular cooling pipes 18.
According to embodiments of the present invention there is provided an electrical machine comprising a winding mounted on a member of the machine, and at least one pre-formed heat-conducting insert having a first part in heat-conducting contact with the winding, and a second part in contact with a heat-dissipating part of the member.
The member may be the stator of an electrical machine on which a winding(s) is wound to energize stator poles. Alternatively, the member may be the moving part of the machine, such as a rotor in a rotary machine, on which a winding(s) is mounted.
The stator, for example, can be actively cooled by the use of one or more cooling pipes arranged around the periphery of the stator body defining the back iron. In this case, the second part of the insert is shaped to make heat-conducting contact with the pipe(s). Alternatively, the second part may be in contact with the body of the stator itself to dissipate the heat drawn by the insert from the winding.
One form of insert has a first part shaped to fill the space between adjacent coils of the winding. In a machine where the space is tapered, the first part is preferably wedge-shaped to fit the taper.
In another form, the insert has a first part that is shaped to lie on the axial end of a stator pole and the winding is placed over the first part so that there is an intimate contact for heat conduction from both the winding and the pole itself. In a stator, the surface of the back iron usually lies flush with the surface of the pole end. Thus, this type of insert preferably has a flat surface that is in contiguous contact with the pole end and the back iron.
When a cooling pipe is used, the second part of the insert can be formed to define a recess in conformity with the pipe section. When a plurality of inserts are present they can be connected by their second parts or arranged so that they form a substantially continuous channel for the pipe.
In one particular form of the invention there is provided a stator for an electrical machine defining a back iron part and a plurality of poles extending from the back iron, the poles having pole sides adjacent one another, a pole face and a pole end between the pole sides, a winding extending around the poles to define a winding overhang at the pole ends, and a pre-formed heat-conducting insert having a first part shaped to fit in heat-conducting contact with the winding overhang, and a second part arranged as a cross-piece to the first part which is in heat-conducting contact with the heat-dissipating part of the stator.
In another particular form of the invention, there is provided a stator for an electrical machine defining a back iron part and a plurality of poles extending from the back iron, a winding arranged in relation to the poles to define spaces between them, and a pre-formed heat-conducting insert having a first part shaped to fit in heat-conducting contact with the winding in the spaces, and a second part arranged as a cross-piece to the first part which is in heat-conducting contact with a heat-dissipating part of the stator.